Spinal fusion is a surgical procedure in which adjacent vertebrae are joined together to prevent abnormal movement. Spinal fusions are used to treat scoliosis, kyphosis, fractures and injury to the vertebrae, symptomatic degenerative disc disease, and spinal instability resulting from tumors or infection.
Currently, there are many different types of metal devices used to perform a lumbar fusion. These devices connect two or more vertebrae together, hold them in the correct position, and keep them from moving until they have a chance to grow together, or fuse. Many of the current devices that surgeons favor use metal screws that are placed through the small tube of bone, called the pedicle, and into the vertebral body. These screws are attached to metal plates or metal rods that are bolted together in the back of the spine. This combination of hardware creates a solid brace that holds the vertebrae in place. These devices are intended to inhibit or at least minimize relative movement of adjacent vertebrae that are being fused, thereby allowing a solid fusion to occur. In many cases bone grafts can be used along with a metal device to assist in the fusion process. Such a metal device provides improved stability to the fusion site and improves fusion of the bones.
Traditionally, bone grafts from the patient's iliac crest (autograft) have been used to fuse adjacent vertebrae with good results, but significant morbidity is often associated with the donor site. As healing occurs, the bone growth creates a solid fusion, which is usually complete within about three months after the procedure and continues to get stronger for one to two years. Bone morphogenetic proteins and other growth factors can be used to promote bone formation. Recently, bone morphogenetic protein (BMP-2) delivered through an absorbable collagen sponge has been found to be effective in osteoinduction and arthrodesis in anterior lumbar interbody fusion and with use of interbody fusion cages. However, the use of the collagen sponge to deliver BMP-2 in posterior intertransverse process fusion is complicated by the tendency of the paraspinal muscles to compress the sponge and allow the BMP-2 to leak away from the fusion site, requiring extremely high doses of BMP-2 to achieve fusion.